FPSO Field Development System for Large Riser Count and High Pressures for Harsh Environments

ABSTRACT

In an offshore production system, a semisubmersible supports the risers and provides the service requirements for the wells, while an associated FPSO provides the processing functions and storage of the produced hydrocarbons. As a result, a greater number of risers can be supported in a harsh environment while the turret and swivel requirements of the system are significantly reduced.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/901,794 filed on Nov. 8, 2013.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to the offshore production ofoil and gas. More particularly, it relates to surface vessels used toreceive, process and store hydrocarbons from subsea wells.

2. Description of the Related Art Including Information Disclosed Under37 CFR 1.97 and 1.98

A Floating Production Storage and Offloading system (“FPSO”) is afloating facility installed above or close to an offshore oil and/or gasfield to receive, process, store and export hydrocarbons.

It consists of a floater, which is either a newly built or convertedtanker, [permanently] moored on site. The cargo capacity of the vesselis used as buffer storage for the oil produced. The process facilities(topsides) and accommodation are installed on the floater. The mooringconfiguration may be of the spread mooring type or a single pointmooring system, generally a turret.

The high pressure mixture of produced fluids is delivered to the processfacilities mounted on the deck of the tanker, where the oil, gas andwater are separated. The water is discharged overboard after treatmentto eliminate hydrocarbons. The stabilized crude oil is stored in thecargo tanks and subsequently transferred into shuttle tankers either viaa buoy or by laying side by side or in tandem to the FPSO. The gas isused for enhancing the liquid production through gas lift, and forenergy production onboard the vessel. The remainder is compressed andtransported by pipeline to shore or re-injected into the reservoir.

A Steel Catenary Riser (“SCR”) is a steel pipe hung in a catenaryconfiguration from a floating vessel in deep water to transmit fluids toor from the seafloor. A Lazy Wave Steel Catenary Riser (“LWSCR”) is anSCR having an elevated intermediate portion that acts to minimize motionof the SCR on the seafloor when the floating vessel is subjected toheave.

BRIEF SUMMARY OF THE INVENTION

In an offshore production system according to the invention, asemisubmersible supports the risers and provides the servicerequirements for the wells, while an FPSO provides the processingfunctions and storage of the produced hydrocarbons. As a result, agreater number of risers can be supported in a harsh environment whilethe turret and swivel requirements of the system are significantlyreduced.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 is a side schematic view of one embodiment of the presentinvention shown in operating conditions.

FIG. 2 is a side schematic view of the embodiment illustrated in FIG. 1during extreme metocean conditions.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a field development methodology foroffshore oil and gas fields in harsh environments (e.g.hurricane/cyclonic) using FPSO technology.

A system according to the invention comprises a semisubmersible, adisconnectable buoy, an FPSO and a bundle of fluid transfer lines. Thesystem is designed to be deployed in deep water harsh environments whereriser performance (either SCR or LWSCR) is a key performance parameter.The power and pressure requirements for gas and/or water injection arekey drivers of having those services on the semisubmersible rather thanbeing, supplied by an FPSO through a swivel, yoke or equivalent system.

The semisubmersible is used to support the risers because of itsfavorable motion characteristics under harsh conditions. Thesemisubmersible also supports the water injection equipment, gasinjection, the High Integrity Protection System (“HIPS”) and anychemical injection equipment. The power for this equipment may begenerated on the semisubmersible. The produced hydrocarbons may bemanifolded into a fluid transfer line. The fluid transfer line may havea predominantly-oil line, a predominantly-gas line and a communicationsline. The pressure in the fluid transfer line is much lower than that ofthe producing wells. The fluid transfer line is connected to a mooredsubmersible buoy. The submersible buoy is connected to an FPSO via abow-mounted turret. All processing and storage is performed on the FPSO.The FPSO may disconnect from the buoy if the environmental conditionsbecome too extreme. The buoy drops below the water surface to a depthwith reduced hydrodynamic loading. The rotational functionality isprovided by the bow-mounted turret. The swivel is mounted onto the FPSO;no rotating parts are on/in the buoy.

In a production system according to the invention, the semisubmersiblesupports the risers and provides the service requirements for the wells,while the FPSO provides the processing and storage of the producedhydrocarbons. As a result, a greater number of risers can be supportedin a harsh environment while the turret and swivel requirements aresignificantly reduced.

A sample schematic of a system according to the invention underoperational conditions is shown in FIG. 1. FIG. 2 is a schematic of themethodology under extreme conditions.

The invention is intended for use in harsh environments. The methodologyis highly applicable to locations where the harsh environments are rarebut intense (a benign methodology might work except for those rareoccasions). Such locations include many of the hurricane/cyclonic areasof the world. Competing systems include the FPSO or semisubmersible byitself, a disconnectable buoy similar to White Rose or Stones or theMoorSpar™ mooring buoy.

The advantages of a system according to the invention include:

-   -   Support for a large number of risers under harsh conditions,        including high pressure, sour service, and high H2S content    -   A HIPS that is located above the water line in a serviceable        environment    -   A water injection system that is located above the water line in        a serviceable environment    -   A fluid transfer system between the semisubmersible and FPSO        that is “low pressure”, and is comprised of only a few lines,        thereby simplifying the design and allowing for more motion.    -   A disconnectable buoy which supports only a few lines and        doesn't require any rotational parts.    -   A swivel requiring only a few flow paths and an electronic data        path    -   A bow-mounted turret, reducing the construction and/or        modification costs of the FPSO and enhancing storage capacity.    -   The entire system is comprised of field-proven hardware    -   Ability to start producing a field before pipeline        infrastructure

Potential disadvantages of such a system include:

-   -   Requirement for two vessels—an FPSO and a semisubmersible, with        associated mooring line infrastructure; and,    -   Operation of two vessels in proximity to each other.

Systems or methodologies that perform similar functions include theFPSO, the semisubmersible, the disconnectable FPSO, the MoorSpar™mooring buoy and riser tower. Each system has its own advantages anddisadvantages. Most of these systems cannot meet at least one of thefollowing requirements: many risers, high pressure, sour service, highH₂ 5 content, large quantities and/or high pressure water injection,harsh environment, storage & offloading.

The independent innovative aspects of the invention include thefollowing:

-   -   A system comprising at least a semi-submersible and FPSO wherein        the processing of the hydrocarbons is not done on the        semisubmersible and the water injection and/or gas injection is        not done on the FPSO;    -   A system that supports deep-water risers in a harsh environment        without an initial pipeline infrastructure;    -   A system that supports deep-water risers that will have to        withstand high H₂ 5 level and/or high temperatures and/or sour        service, without an initial pipeline infrastructure; and,    -   Use of one or more disconnectable FPSOs in conjunction with a        semisubmersible.

Although particular embodiments of the present invention have been shownand described, they are not intended to limit what this patent covers.One skilled in the art will understand that various changes andmodifications may be made without departing from the scope of thepresent invention as literally and equivalently covered by the followingclaims.

What is claimed is:
 1. An offshore oilfield development system comprising: a semisubmersible having equipment for water and/or gas injection, chemical injection, and manifolding; a plurality of subsea risers in fluid connection with the equipment on the semisubmersible and wellheads on the seafloor; and a submersible mooring buoy having means for mooring an FPSO said means including means for fluid transfer from the mooring buoy to an FPSO moored thereto; and at least one fluid conduit connecting the mooring buoy to the subsea risers via the semisubmersible. 